U.S. patent number 6,363,255 [Application Number 09/411,073] was granted by the patent office on 2002-03-26 for mobile communications system and mobile station therefor.
This patent grant is currently assigned to Fujitsu Limited. Invention is credited to Soichi Kuwahara.
United States Patent |
6,363,255 |
Kuwahara |
March 26, 2002 |
Mobile communications system and mobile station therefor
Abstract
A mobile communications system having a mechanism to reduce the
control message traffic in paging and location updating operations.
Each mobile station receives from the nearest base station a
notification message containing its base station ID. A location
change counting unit detects a change in the mobile location by
comparing two consecutive instances of the base station ID. It
counts and records such location changes, together with the two
base station IDs involved in each change. Based on the location
change count values, a personal location area (PLA) setting unit
defines personal location areas by grouping associated cells into
one area. A PLA memory stores the defined personal location areas
and relevant base station IDs. Each time a new base station ID is
detected, a location change detector tests whether the presently
registered personal location area is still valid. If it is no
longer valid, a location registration unit searches the PLA memory
for a new relevant personal location area. The location
registration unit then sends a location updating message to notify
the mobile switching center of the new personal location area and
its relevant base station IDs.
Inventors: |
Kuwahara; Soichi (Fukuoka,
JP) |
Assignee: |
Fujitsu Limited (Kawasaki,
JP)
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Family
ID: |
17922108 |
Appl.
No.: |
09/411,073 |
Filed: |
October 4, 1999 |
Foreign Application Priority Data
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Oct 26, 1998 [JP] |
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10-303530 |
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Current U.S.
Class: |
455/456.5;
455/435.1; 455/458 |
Current CPC
Class: |
H04W
60/04 (20130101); H04W 68/04 (20130101); H04W
68/06 (20130101) |
Current International
Class: |
H04Q
7/38 (20060101); H04Q 007/20 () |
Field of
Search: |
;455/422,423,425,432,433,434,435,436,439,440,450,452,456-457,458-464,517,519
;370/336,335,331 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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3-85828 |
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Apr 1991 |
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JP |
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4-220821 |
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Aug 1992 |
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JP |
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9-27985 |
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Jan 1997 |
|
JP |
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9-116952 |
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May 1997 |
|
JP |
|
Primary Examiner: Legree; Tracy
Assistant Examiner: Gelin; Jean A
Attorney, Agent or Firm: Rosenman & Colin LLP
Claims
What is claimed is:
1. A mobile communications system where base stations provide radio
links for mobile stations in each cell and a mobile switching
center controls connections between the mobile stations by
switching circuits, the system comprising: notification message
reception means, disposed in each mobile station, for receiving a
notification message from the nearest base station; base station ID
extracting means, disposed in each mobile station, for extracting a
base station ID from the received notification message to identify
which base station is sending the notification message; location
change counting means, disposed in each mobile station, for
detecting location changes by comparing two consecutive instances
of the base station IDs extracted by said base station ID
extracting means, and counting the detected location changes
separately for each pair of base station IDs involved in the
changes; personal location area (PLA) setting means, disposed in
each mobile station, for defining personal location areas by
grouping associated cells into one area, based on the location
change count values obtained by said location change counting
means; . PLA memory means, disposed in each mobile station, for
storing records of the personal location areas defined by said PLA
setting means, each of the records containing the base station IDs
indicating which cells constitute each personal location area;
location change detecting means, disposed in each mobile station,
for testing whether the personal location area registered at
present is valid, each time said base station ID extracting means
extracts a base station ID that is different from before; location
registration means, disposed in each mobile station and activated
when said location change detecting means has detected that the
registered personal location area is no longer valid, for searching
said PLA memory means to retrieve one of the records of personal
location areas that contains the extracted base station ID, and
performing location registration by sending to the mobile switching
center the retrieved personal location area and base station IDs
relevant thereto; PLA configuration data storage means, disposed in
the mobile switching center, for storing records of the personal
location area and relevant base station IDs received from each
mobile station; and paging means, disposed in the mobile switching
center and responsive to an incoming call to one of the mobile
stations, for searching said PLA configuration data storage means
to retrieve one of the records that is relevant to the called
mobile station, and paging the called mobile station through the
base stations specified in the retrieved record.
2. The mobile communications system according to claim 1, wherein
said personal location area setting means extracts such mobile
station ID pairs that exhibit larger location change count values
exceeding a predetermined threshold, and defining the personal
location areas by grouping the mobile station ID pairs having a
predetermined relationship.
3. The mobile communications system according to claim 1, wherein
said location registration means performs location registration by
using network default location areas that the system provides, if
no relevant personal location area is found in said PLA memory
means.
4. The mobile communications system according to claim 1, wherein:
each of the mobile stations comprises real-time clock means for
providing date and time information, and said PLA setting means
refers to the date and time information provided by said real-time
clock means to update the definitions of the personal location
areas at predetermined intervals.
5. The mobile communications system according to claim 1, wherein:
said mobile switching center further comprises minimum/maximum
number notification means for notifying the mobile stations of
parameters specifying suitable size of each personal location area,
the parameters including minimum and maximum numbers of cells to be
grouped into one personal location area; and said PLA setting means
uses the minimum and maximum numbers of cells to define the
personal location areas.
6. The mobile communications system according to claim 5, wherein:
said mobile switching means further comprises incoming call
frequency memory means for counting incoming calls addressed to
each individual mobile station; and said minimum/maximum number
notification means modifies the minimum and maximum numbers,
referring to the count values provided by said incoming call
frequency memory means, in such a way that the minimum number or
maximum number for frequently-called mobile stations will be
reduced by a predetermined number, while the minimum number or
maximum number for infrequently-called mobile stations will be
increased by another predetermined number.
7. The mobile communications system according to claim 5, wherein:
each of the mobile stations further comprises incoming call
frequency memory means for counting incoming calls addressed
thereto; and said PLA setting means modifies minimum and maximum
numbers of cells to be grouped, referring to count records provided
by said incoming call frequency memory means, in such a way that
the minimum number or maximum number for frequently-called mobile
stations will be reduced by a predetermined number, while the
minimum number or maximum number for infrequently-called mobile
stations will be increased by another predetermined number.
8. The mobile communications system according to claim 1, wherein
each of the mobile stations further comprises disabling means for
disabling said location change counting means and said PLA setting
means.
9. The mobile communications system according to claim 8, wherein
each of the mobile stations further comprises overriding means for
temporarily overriding said disabling means to make said location
change counting means and said PLA setting means operate, and if a
different personal location area is obtained during the temporary
overriding, continuing to override said disabling means for a
predetermined extended period.
10. The mobile communications system according to claim 1, wherein
each of the mobile stations further comprises disabling means for
disabling said location change counting means and said PLA setting
means when said each mobile station is roaming.
11. The mobile communications system according to claim 1, wherein
the mobile stations communicate with the mobile switching center by
using CDMA techniques.
12. A mobile station for use in a mobile communications system
where base stations provide radio links for mobile subscribers in
each cell and a mobile switching center controls connections
between the mobile subscribers by switching circuits, the mobile
station comprising: notification message reception means for
receiving a notification message from the nearest base station;
base station ID extracting means for extracting a base station ID
from the received notification message to identify which base
station is sending the notification message; location change
counting means for detecting location changes by comparing two
consecutive instances of the base station IDs extracted by said
base station ID extracting means, and counting the detected
location changes separately for each pair of base station IDs
involved in the changes; personal location area (PLA) setting
means, disposed in each mobile station, for defining personal
location areas by grouping associated cells into one area, based on
the location change count values obtained by said location change
counting means; PLA memory means, disposed in each mobile station,
for storing records of the personal location areas defined by said
PLA setting means, each of the records containing the base station
IDs indicating which cells constitute each personal location area;
location change detecting means for testing whether the personal
location area registered at present is valid, each time said base
station ID extracting means extracts a base station ID that is
different from before; and location registration means, disposed in
each mobile station and activated when said location change
detecting means has detected that the registered personal location
area is no longer valid, for searching said PLA memory means to
retrieve one of the records of personal location areas that
contains the extracted base station ID, and performing location
registration by sending to the mobile switching center the
retrieved personal location area and base station IDs relevant
thereto.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mobile communications system and
mobile stations therefor. More particularly, the invention relates
to a mobile communications system with mobility management
functions, including location updating operations, which permit
mobile switching centers to keep track of mobile stations roaming
between different cells. The invention also to the structure of a
mobile station for use in the system.
2. Description of the Related Art
The major components in a typical mobile communications system
include: (a) mobile stations, or portable handsets carried by
mobile users; (b) a plurality of base stations which support their
respective radio service coverage areas, or cells; and (c) a mobile
switching center which controls these base stations. When a mobile
station places a call to another mobile station, this call request
reaches the mobile switching center via a base station nearest to
the calling mobile station. The mobile switching center, which
tracks the location of every mobile station, sends a paging request
message to another base station that is currently serving the
called station. The base station transmits this message toward the
called mobile station. If a paging response is returned, the system
establishes a traffic channel between the calling station and
called station, making it possible to transport voice or data
signals.
Tasks of keeping track of the location of each mobile station are
known as "mobility management," which typically treats several
adjacent cells as one unit for management purposes. The areas
consolidated as such are called "location areas." FIG. 14 explains
the concept of this location area, where smaller circles represent
individual cells served by base stations named BS1 to BS40. Larger
circles each including seven smaller circles denote location areas
defined in this system.
Base stations are designed to transmit the identifier of the
location area they belong to, thus allowing mobile stations within
the area to recognize which location area they are currently
visiting. The mobile stations update their locations by sending a
location registration (or location update) message to the mobile
switching center, when they move between different location areas.
The mobile switching center records this information in its local
and remote databases known as the home location register (HLR) and
visitor location register (VLR). When there is an incoming call
addressed to a specific mobile station, the mobile switching center
first identifies the called station's current location area, and
then directs all the base stations in that area to transmit a
paging request message.
Determining the size of a location area is not a simple task,
because there are two conflicting factors that should be balanced.
They are: paging message traffic and location update message
traffic. If cells are grouped into smaller location areas, the
traffic of paging request messages will decrease, because the
number of base stations that simultaneously send a paging request
message is reduced. However, this also means that mobile stations
have to cross the border between location areas more often, thus
sending an increased number of location update messages in total.
On the other hand, larger location areas will reduce the total
number of location updating messages, but instead, increase the
paging messages traffic. Therefore, the system designer has to make
a compromise between paging message traffic and location update
message traffic, so as to reach the optimal location area size.
Another issue concerning location area design is that mobile users
have their own activity patterns. They visit different places at
different frequencies. For this reason, a specific setup optimized
for one user may not always work effectively with another user.
Researchers in this technical field have proposed various solutions
for this problem as explained below.
One example is the Japanese Patent Application Laid-open
Publication No. 8-84364 (1996), regarding a paging system for
cellular telephone networks. This patent application proposes a
special process to reduce the location updating traffic in such a
network where a mobile station frequently moves between two
location areas MLA1 and MLA2 shown in FIG. 15. First, the logs of
outgoing and incoming calls are used to figure out which cells this
mobile station is likely to visit, thereby identifying several
frequently-visited cells. Then, the base stations serving those
cells are grouped into what is called a "virtual mobile location
area (VMLA)." Referring to the example of FIG. 15, the VLMA
involves three base stations BS12, BS16, and BS34. The mobile
station registers this VLMA as its new location area.
Because the registered location area contains the cells the mobile
station often visit, the necessity of location updating will be
effectively reduced. About paging operations, the proposed system
regards the base stations belonging to VMLA as the primary paging
group, the rest of MLA1 as the secondary paging group, and the rest
of MLA2 as the tertiary paging group. When a call is placed, the
paging operation is attempted first in the primary paging group,
then in the secondary group, and lastly in the tertiary group. In
this way, the proposed system starts to page the mobile station
from its VMLA, which is supposed to be the most likely place. As
long as the number of base stations in the VMLA is less than in
other areas such as MLA1 and MLA2, the proposed paging system will
effectively reduce the amount of paging message traffic.
The above paging system, however, would not work as expected, when
the mobile station tends to roam through a wide geographical area,
or when each location area has to cover a relatively large number
of cells as a result of introduction of microcells. In such
situations, a larger VMLA would cause more paging messages to be
transmitted. However, this is not to say that the system would work
with a smaller VMLA, because the mobile station is less likely to
respond to the first attempt of paging in a small area. When the
intended mobile station is visiting outside the VMLA, the system is
unable to connect with it immediately. This is because the paging
process is programmed to start from the VMLA, and thus it takes
some time to route the paging request to the right area.
Another solution is proposed in the Japanese Patent Application
Laid-open Publication No. 9-116952 (1997), disclosing a mobile
communications system. FIG. 16 is a diagram showing this proposed
system, where the hatched circles represent the cells visited by a
specific subscriber A, and the bold circles show those of another
subscriber B. Location areas are defined on the basis of such
activity pattern data of each subscriber, and mobile stations in
this system are designed not to update their location register
unless they leave their respective location areas, thus reducing
the location update message traffic.
To describe the above mechanism more specifically, each mobile
station measures how long it has been staying in a specific cell,
and notifies the mobile switching center of the duration. If the
notified duration is longer than a predetermined threshold, the
mobile switching center registers the serving base station as a
member base station of the location area being developed. When an
incoming call to that mobile station is placed, the mobile
switching center pages the station through all the member base
stations within the location area. As such, the proposed system
defines location areas according to each mobile station's mobility
pattern to avoid any possible increase in the location updating
traffic.
Some active subscribers may visit too many cells to fit in a single
location area. If this is the case, the mobile switching center
evaluates the duration of their stay in each cell and sorts the
relevant cells in descending order of the likelihood of their
presence. In the example of FIG. 16, the subscriber A's presence is
classified into three levels (and represented by three different
symbols) as follows: high level (small black circles), medium level
(double circles), and low level (white circles). According to those
levels, the cells are grouped into larger areas for paging
purposes. When a call to this mobile station is detected, the
mobile switching center first directs the high-level group of base
stations to page the station. If no paging response is obtained
there, it then commands the medium-level group to do the same. If
it is still unable to hear the response, the paging request is
routed to the low-level group. In this way, the proposed system
offers a wide paging area, while taking advantage of the high
likelihood of mobile presence. This makes it possible to suppress
the paging message traffic, even when the mobile station tends to
roam through a wide geographical area.
Mobile stations store all base station IDs relevant to their
respective location areas. Suppose that one mobile station is
leaving its location area. The mobile station will soon detect the
departure by itself, because the base station ID received from the
nearest base station does not coincide with any of the base station
IDs they have. The mobile station now attempts to register its new
location by using an ordinary system of location areas (i.e., those
shown in FIG. 14), in order to keep on operating properly.
The above-described mobile communications system works fine as far
as the activity of each mobile station is limited within a
relatively small geographical region. However, this may not apply
to such a subscriber who commutes to his/her distant office. In
this case, two isolated location areas will be produced: one around
his/her home, and the other around the office. Now, assume that
there arises a call to his/her mobile station located in one
location area with a lower probability of presence. Paging is
attempted, however, in the other location area which is supposed to
have a higher probability of presence. This attempt should end up
with no response, only wasting time and bandwidth.
Again, the system is designed to combine most likely cells where a
specific subscriber is expected to be found at a higher
probability. However, it may not always be true that such cells are
located closely to each other. With isolated location areas,
location registration operations may not be performed
efficiently.
SUMMARY OF THE INVENTION
Taking the above into consideration, an object of the present
invention is to provide a mobile communications system which
effectively suppresses the control message traffic when updating
location registers and paging a specific mobile station.
To accomplish the above object, according to the present invention,
there is provided a mobile communications system where base
stations provide radio links for mobile stations in each cell and a
mobile switching center controls connections between the mobile
stations by switching circuits. In this system, each mobile station
comprises the following element: (1a) a notification message
receiver which receives a notification message from the nearest
base station; (1b) a base station ID extractor which extracts a
base station ID from the received notification message to identify
which base station is sending the notification message; (1c) a
location change counting unit which detects a change in the mobile
location by comparing two consecutive instances of the base station
ID extracted by the base station ID extractor, and counts the
detected location changes separately for each pair of base station
IDs involved in each change; (1d) a personal location area (PLA)
setting unit which defines personal location areas by grouping
associated cells into one area, based on the location change count
values obtained by the location change counting unit; (1e) a PLA
memory which stores records of the personal location areas defined
by the PLA setting unit, each of the records containing the base
station IDs indicating which cells constitute each personal
location area; (1f) a location change detector which tests whether
the personal location area registered at present is valid, each
time the base station ID extracting unit extracts a base station ID
that is different from before; and (1g) a location registration
unit which is activated when the location change detector has
detected that the registered personal location area is no longer
valid. It searches the PLA memory to retrieve one of the records of
personal location areas that contains the extracted base station
ID, and performs location registration by sending to the mobile
switching center the retrieved personal location area and its
relevant base station IDs.
Further, according to the proposed system, the mobile switching
center comprises the following elements: (2a) a PLA configuration
data memory which stores records of the personal location area and
relevant base station IDs received from each mobile station; and
(2b) a paging unit which searches the PLA configuration data
memory, in response to an incoming call to one of the mobile
stations, to retrieve one of the records that is relevant to the
called mobile station, and pages the called mobile station through
the base stations specified in the retrieved record.
The above and other objects, features and advantages of the present
invention will become apparent from the following description when
taken in conjunction with the accompanying drawings which
illustrate preferred embodiments of the present invention by way of
example.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a mobile communications system
according to an embodiment of the present invention;
FIG. 2 is a diagram which illustrates personal location areas
defined by a mobile station in the system of FIG. 1;
FIG. 3 is a diagram which shows a typical location change counting
table created in the mobile station;
FIG. 4 is a flowchart which shows a typical process executed to
define personal location areas in the system of FIG. 1;
FIG. 5 is an example of a base station list produced at step S3 in
the flowchart of FIG. 4;
FIG. 6 is a diagram which shows a notification message transmitted
from base stations to mobile stations in the system of FIG. 1;
FIG. 7 is a diagram showing a list of personal location areas
obtained by applying the process of FIG. 4 to the personal location
area of FIG. 2:
FIG. 8 is a flowchart which shows the details of "MERGE" routine
called at step S7 in the flowchart of FIG. 4;
FIG. 9 is a diagram which shows the cells visited by the mobile
station;
FIG. 10 is a flowchart which shows a typical process executed by
the mobile station to register its location;
FIG. 11 is a flowchart which shows the details of a "LOCATION
REGISTRATION" routine called at step S49 in the flowchart of FIG.
9;
FIG. 12 is a diagram showing an example of a location update
message that the mobile station sends to the mobile switching
center when registering its personal location area;
FIG. 13 is a diagram showing another example of the location update
message;
FIG. 14 is a diagram which explains the concept of location areas
in a conventional mobile communications system;
FIG. 15 is a diagram which shows typical location areas in a
conventional mobile communications system; and
FIG. 16 is a diagram which shows another example of location areas
in a conventional mobile communications system.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will be described
below with reference to accompanying drawings.
FIG. 1 is a block diagram of a mobile communications system
according to the present invention. Broadly, this system comprises
a mobile station 1, a mobile switching center 2, a network 3, and
base stations 4-1 to 4-n. The mobile station 1 comprises the
following elements: a notification message receiver 1a, a base
station ID extractor 1b, a location change counter 1c, a PLA
setting unit 1d, a PLA memory 1e, a location change detector 1f, a
location registration unit 1g, a real-time clock 1h, a first
disabling unit 1i, an overriding unit 1j, and a second disabling
unit 1k. Those elements provide the functions described below.
The notification message receiver 1a receives notification messages
from the nearest base station, which include, for example, a base
stations ID to identify the sending base station. The base station
ID extractor 1b extracts this base station ID from the received
notification message. The location change counting unit 1c detects
a change in the mobile location by comparing two consecutive
instances of the base station ID extracted by the base station ID
extractor 1b. When the base station ID received this time is not
identical with the previous one, the location change counting unit
1c stores a record of the two base station IDs, as well as counting
this event.
Based on the above data accumulated by the location change counting
unit 1c, the PLA setting unit 1d defines personal location areas
(PLAs) by dividing the cells of the base stations 4-1 to 4-n into
appropriate groups. The PLA memory 1e stores the resultant personal
location areas, together with their associated base station
IDs.
The location change detector if keeps track of the base station IDs
detected by the base station ID extractor 1b to check the validity
of the personal location area that is registered at present. That
is, each time a new base station ID is detected, it tests whether
the mobile station 1 has left the present personal location area.
If this test has revealed that the current location registration is
no longer valid, the location registration unit 1g searches the PLA
memory 1e to find a new personal location area that includes the
new base station as its member. It then informs the mobile
switching center 2 of the new personal location area, thereby
updating the mobile location.
The real-time clock 1h provides date and time information to the
PLA setting unit 1d and other elements of the mobile station 1. The
first disabling unit 1i stops the operation of the location change
counting unit 1c and PLA setting unit 1d to reduce workloads and
save energy. The overriding unit 1 temporarily enables the
operation to resume even when the first disabling unit 1i is
active. This gives the mobile station 1 an opportunity to review
the validity of the present definition of personal location areas.
If the personal location areas obtained during this period do not
agree with those used at present, this indicates that the present
setup has lost validity. If this is the case, the overriding unit
1j further overrides the first disabling unit 1i for a
predetermined extended period, allowing the personal location areas
to be refreshed. The second disabling unit 1k stops the location
change counting unit 1c and PLA setting unit 1d when the mobile
station 1 is roaming. Here, the term "roaming" denotes that the
mobile station has left its home location and is visiting a remote
cell.
On the other hand, the mobile switching center 2 comprises the
following elements: a PLA configuration data memory 2a, a paging
unit 2b, an incoming call frequency memory 2c, and a
minimum/maximum number notification unit 2d. These elements are
designed to operate as follows.
The PLA configuration data memory 2a stores personal location areas
defined by, and informed from, each mobile station 1, together with
their associated base station IDs. The paging unit 2b pages mobile
stations when they are called. That is, when there is an incoming
call addressed to a particular mobile station, the paging unit 2b
searches the PLA configuration data memory 2a to obtain relevant
base station IDs and sends a paging request message through the
base stations identified by those IDs.
The incoming call frequency memory 2c stores records of how many
incoming calls each mobile station has received. It provides the
minimum/maximum number notification unit 2d with such records, if
requested. The minimum/maximum number notification unit 2d notifies
the mobile station 1 of the minimum number and maximum number of
cells to be grouped into one personal location area. Those minimum
and maximum numbers, which actually limit the size of personal
location areas, are determined by the mobile switching center 2 so
as to optimize the system performance. If appropriate, the
minimum/maximum number notification unit 2d changes those
parameters, referring to the records of incoming calls stored in
the incoming call frequency memory 2c, and notifies the mobile
station 1 of the new parameter values. (The above maximum and
minimum numbers will appear in a later section as the maximum
allowable number of base stations and minimum required number of
base stations, respectively. Note that, in the present context, the
number of cells is equivalent to the number of base stations.)
The next section will now describe how the system operates in the
above-described embodiment of the present invention. For
illustrative purposes, it is assumed that there is a service
coverage area comprising forty cells as indicated by small circles
in FIG. 2. These cells are supported by base stations BS1 to BS40
under the control of the mobile switching center 2 (FIG. 1). Some
cells in FIG. 2 are hatched with slant lines, indicating that they
are visited relatively frequently by subscriber A, the user of the
mobile station 1 (FIG. 1).
Consider, for example, that the mobile station 1 is located in the
cell that the base station BS1 serves. The base station BS1
transmits its own base station ID in a notification message, which
is received by the notification message receiver 1a of the mobile
station 1. The base station ID extractor 1b extracts this base
station ID from the received notification message and supplies it
to the location change counting unit 1c. Each time a different base
station ID is received from the base station ID extractor 1b (that
is, each time the mobile station 1 enters a different cell), the
location change counting unit 1c increments a corresponding item of
its internal location change counting table as will be described
below.
FIG. 3 shows the structure of the location change counting table,
which lists all available pairs of adjacent base stations in matrix
form. Each element of this matrix indicates how many times the
mobile station 1 has changed its location from one particular cell
to another particular cell. Note that the lower left half of this
table is left blank because the direction of movements is
disregarded. Because of space limitation, FIG. 3 is unable to show
the table in its entirety, but only presents BS1 to BS15 out of
forty base stations. Take the element at the upper left corner of
this table for example. This element indicates that five
transitions from BS1 to BS2, or BS2 to BS1, were observed, meaning
that the mobile station 1 moved five times from the cell of BS1 to
that of BS2, or vice versa.
The location change counting table will grow up to a statistically
meaningful level, after such events are accumulated for a
predetermined period (e.g., one month). The mobile station 1 now
executes a process to define its personal location areas according
to a flowchart of FIG. 4. This process comprises the following
steps.
(S1) The location change counting unit 1c normalizes the data in
the location change counting table of FIG. 2. More specifically, it
first extracts a maximum value from the table and then divides all
the items by this maximum value. This operation will make all the
count values in the table fall within the range of zero to one,
hence normalizes the table data.
(S2) The location change counting unit 1c scans the normalized
table to pick up such location change count values that exceed a
predetermined threshold (e.g., 0.1).
(S3) The PLA setting unit 1d extracts base station IDs relevant to
the location change count values picked up at step S2, thereby
creating a list of base stations. In the present example of FIG. 2,
the list includes the following base stations: BS1, BS6, BS7, BS12,
BS13, BS19, BS24, BS29, BS33, and BS38. This step S3 results in a
base station group shown in FIG. 5.
(S4) The PLA setting unit 1d initializes a variable i to one.
(S5) The PLA setting unit 1d extracts a base station pair (BSj,
BSk) corresponding to the i-th largest location change count. Here,
the expression (BSJ, BSk) refers to a pair of base stations (or
base station IDs, more precisely) BSJ and BSk, which the mobile
station 1 may move from one to the other. According to the table of
FIG. 3, the base station pair (BS1, BS6) exhibits the largest
count, namely 98. The PLA setting unit 1d thus chooses this pair
(BS1, BS6) in the first iterative cycle, where i=1.
(S6) The PLA setting unit 1d tests whether either one of the two
base stations (BSJ, BSk) extracted at step S5 has already been
registered in a PLA table. This PLA table is a table held in the
PLA memory 1e to store the definition of personal location areas
created through the present process. If either one is found in the
PLA table, the process advances to step S7. If neither of them is
found, it proceeds to step S8.
(S7) The PLA setting unit 1d calls a "Merge" routine to combine the
extracted base station pair (BSj, BSk) with the relevant PLA entry
by using their common base station ID as a key. The full details of
this merging process will be explained later, with reference to a
dedicated flowchart.
(S8) The PLA setting unit 1d registers the base station pair (BSJ,
BSk) as a new entry to the PLA table in the PLA memory 1e.
(S9) The PLA setting unit 1d determines whether any unprocessed
base station pairs still exist. If any such pairs are found, the
process proceeds to step S10. If all the pairs are finished, the
process advances to step S11.
(S10) The PLA setting unit 1d increments the variable i by one. The
process then returns to step S5.
(S11) The PLA setting unit 1d extracts such an entry of the PLA
table that has not reached the minimum number of base stations.
Here, the minimum number of base stations is one of the parameters
given by the mobile switching center 2, which actually means the
minimum required number of cells that each personal location area
must include. The minimum/maximum number notification unit 2d
supplies this parameter to the base stations BS1 to BSn, which then
distribute it in a notification message addressed to mobile
stations in their respective cells. As FIG. 6 shows, the
notification message conveys the following information: network
default location area ID provided by the system, base station ID of
the sending base station, maximum number of base stations to be
grouped, minimum number of base stations to be grouped, and so
on.
(S12) The PLA setting unit 1d reinitializes the variable i to
one.
(S13) The PLA setting unit 1d searches the location change counting
table (FIG. 3) for a base station pair that exhibits the i-th
largest count value. If the retrieved pair is related to the PLA
entry of interest, the PLA setting unit 1d then merges the pair
with the PLA entry. More specifically, if either one of the
retrieved base station pair is found in the PLA entry as its member
base station, the other one of the pair will be added to the PLA
entry as a new member.
(S14) The PLA setting unit 1d determines whether the member base
stations registered in the PLA entry of interest has reached the
minimum required number of base stations. If the PLA entry
satisfies this condition, the process advances to step S16, and
otherwise, the process proceeds to step S15.
(S15) The PLA setting unit 1d increments the varible i by one. The
process then returns to step S13.
(S16) The PLA setting unit 1d tests whether all the entries in the
PLA table satisfy the requirement in terms of the minimum number of
base stations. If they satisfy the requirement, the process
terminates. Otherwise, the process returns to step S11 for the next
iteration.
FIG. 7 shows a list of personal location areas which result from
the above process when it is applied to the mobility pattern shown
in FIG. 2. The list contains four such areas PLA1 to PLA4, which
are represented by bold circles in FIG. 2. Take the first personal
location area PLA1 for example. This PLA1 involves three registered
base stations, BS1, BS6, and BS7, which govern adjacent three
cells. FIG. 2 also shows different mobility levels by three kinds
of line symbols. First, the bold solid line segments each
connecting two specific cells indicate a high mobility, meaning
that the mobile station 1 moves from one cell to the other
relatively often (i.e., frequent handoff operations would occur
between those two base stations). Then the broken bold line
segments denote an average mobility, and the thin line segments a
low mobility.
Referring next to FIG. 8, the details of the "Merge" routine, which
appeared at step S7 in the flowchart of FIG. 4, will be explained
below. This routine comprises the following steps. Note that a
specific base station pair (BS, BSk) extracted at step S5 is passed
to this routine as an argument.
(S20) The PLA setting unit 1d tests whether the PLA table has two
different entries that include the given base station IDs, BSj and
BSk, respectively. If this test reveals that there exist two such
entries, the process advances to step S21. If there is only one
such entry, it proceeds to step S25.
Suppose, for instance, that a specific base station pair (BS1, BS2)
was extracted at step S5, while the present entries of the PLA
table include (BS1, BS4) and (BS2, BS5). Then the process would
proceed to step S21, because one element BS1 of the pair is common
to the former PLA entry (BS1, BS4), and the other element BS2 is
common to the latter PLA entry (BS2, BS5).
(S21) The PLA setting unit 1d retrieves those two entries from the
PLA table. In the above example, the entries (BS1, BS4) and (BS2,
BS5) are retrieved.
(S22) The PLA setting unit 1d then calculates the number of base
stations included in either the base station pair extracted at step
S5 or the entries retrieved at step S21. Note that any duplicated
instances should be rejected from the sum total. It then compares
the total number of base stations with a given parameter, the
maximum number of base stations. If the calculated total number is
not greater than this parameter, the process advances to step S23.
Otherwise, it goes to step S27.
The maximum number of base stations denotes the number of cells
that can be grouped into one personal location area. In other
words, it sets an upper limit on the size of a personal location
area. The minimum/maximum number notification unit 2d supplies this
parameter to the base stations BS1 to BSn, which then distribute it
to mobile stations in their respective cells, in the form of a
notification message shown in FIG. 6. In the present example, the
two PLA entries and extracted base station pair involve four base
stations in total. When the maximum number of base stations is set
to four or more, the process will proceed to step S23.
(S23) The PLA setting unit 1d merges the given base station pair
with the two entries retrieved at step S21. In the present example,
the given base station pair (BS1, BS2) and the two entries (BS1,
BS4) and (BS2, BS5) are merged into one group, thus yielding a new
expanded entry (BS1, BS2, BS4, BS5).
(S24) The PLA setting unit 1d replaces one of the two entries
retrieved at step S21 with the new entry and deletes the other
entry, thereby consolidating the two entries into one. In the
present example, the newly produced entry (BS1, BS2, BS4, BS5) is
stored into a memory space where the old entry (BS1, BS4) was held,
and the other old entry (BS2, BS5) is removed from the PLA
table.
(S25) The PLA setting unit 1d calculates the number of base
stations included in either of the base station pair and entry in
process. Note again that any duplicated instances should be
rejected from the sum total. It then compares the total number of
base stations with the maximum number of base stations. If the
calculated total number is not greater than the maximum number, the
process advances to step S26, and otherwise, it goes to step
S27.
Suppose, for instance, that the present entries of the PLA table
include only one relevant entry (BS1, BS4) to the base station pair
of interest. Since the test at step S20 fails, the process proceeds
to step S25, where the total number of base stations is compared
with the maximum allowable number. In the present example, the base
station pair and its relevant PLA entry involve three base stations
in total. If the maximum allowable number of base stations is set
to three or more, the process will proceed to step S26.
(S26) The PLA setting unit 1d merges the given base station pair
with the PLA entry of interest. In the present example, a new entry
(BS1, BS2, BS4) is created from the given base station pair and its
relevant PLA entry, and the PLA table is updated by replacing the
existing entry (BS1, BS4) with the newly created entry.
(S27) The PLA setting unit 1d registers the given base station pair
(BSj, BSk) to the PLA table as a new entry. This step is executed
when step S22 or S26 has revealed that the total number of base
stations would exceed the maximum allowable number if a merge was
performed.
The above-described processes yield several personal location
areas, reflecting the past behavior of a mobile user. FIG. 9 shows
an example of a mobility pattern of the mobile station 1, in which
small circles represent the cells that the user has visited in the
past, and line segments interconnecting them indicate different
mobility levels. That is, the bold solid line segments denote that
the mobile station 1 moves between the two cells relatively often.
The bold broken line segments indicate average frequencies, and the
thin line segments represent relatively infrequent movement. In
this example, the above-described process forms a personal location
area from, for example, cells #1, #6, and #7, since those cells
exhibit relatively high levels of user movement.
If necessary, the user of the mobile station 1 can stop the
operation of the base station ID extractor 1b and PLA setting unit
1d by activating the first disabling unit 1i to keep the current
setting of personal location areas. This feature may be useful
when, for example, the owner of the mobile station 1 lends his/her
handset to another person. Notice that the definition of the
personal location areas are optimized for the owner's mobility
pattern. The first disabling unit ii will permit him/her to protect
that valuable data from being altered.
In addition to the above usage of the first disabling unit 1i, it
is also possible to configure the mobile station 1 to activate the
first disabling unit 1i automatically, each time a complete set of
personal location areas is established (i.e., after the expiration
of a one-month monitoring period). This option is based on such an
assumption that the user's mobility pattern is likely to be stable
and there would thus be no immediate need to update the setup. By
disabling the PLA setting unit 1d and related functions, the mobile
station 1 reduces its processing load and power consumption.
As an additional feature of the proposed mobile station 1, the
overriding unit 1j suppresses the above functions of the first
disabling unit 1i temporarily, allowing a new set of personal
location areas to be obtained intermittently for validity checking
purposes. The PLA setting unit 1d compares the newly obtained
personal location areas with those registered in the PLA memory 1e,
and if they exhibit substantial variations enough to invalidate the
current setup, it then refreshes the PLA entries with the new ones.
This feature permits the mobile station 1 to maintain the validity
of personal location areas, tracking possible variations in the
user's mobility pattern.
Meanwhile, recall that the maximum and minimum numbers of base
stations are specified by the mobile switching center 2 and
delivered as part of the notification message (FIG. 6) sent from
the nearest base station. These parameters are actually determined
on the basis of statistics about how often the mobile station
receives incoming calls. As mentioned earlier, the frequency of
incoming calls to each mobile station is monitored by the mobile
switching center 2 and is recorded in its incoming call frequency
memory 2c. These records are used to extract two particular groups
of mobile stations: frequently-called stations and
infrequently-called stations. Where appropriate, the
minimum/maximum number notification unit 2d assigns smaller numbers
to the maximum and minimum numbers of base stations for use by
frequently-called stations, thereby reducing the size of each
personal location area they will create. For infrequently-called
mobile stations, on the other hand, it assigns larger numbers to
those two parameters, allowing them to create larger personal
location areas.
The above system provides the following benefit. First, it is
possible to suppress the expected increase in paging message
traffic by narrowing down the personal location areas of
frequently-called mobile stations. Also, it is possible to suppress
the location registration message traffic by enlarging the personal
location areas of infrequently-called mobile stations.
Further, as a variation of the above-described mechanism, the
mobile station 1 may record for itself how many times it has been
paged, instead of counting the calls at the mobile switching center
2. In this case, the mobile station 1 may, if needed, modify the
maximum and minimum numbers of base stations after receiving them
as part a notification message.
Referring now to a flowchart of FIG. 10, the following section will
describe an example of a process whereby the mobile station 1
checks and registers its current location. This process will be
executed, each time the base station ID extractor 1b detects a new
base station ID, according to the following steps.
(S40) The second disabling unit 1k tests whether the mobile station
1 is currently roaming. If it is roaming, the process advances to
step S41, and otherwise, the process proceeds to step S42. Here,
the term "roaming" is used to mean such a situation where the
mobile station 1 visits a remote cell that is far from its home
location. To avoid possible accumulation of unnecessary data, the
mobile station 1 may need to stop its internal process of setting
personal location areas while it is in roaming mode. The second
disabling unit 1k provides this option, examining notification
messages from the serving base station to detect an in-roaming
status.
(S41) The second disabling unit 1k tests whether the
registration-in-roaming flag is set. If it is set, the process
advances to step S42. Otherwise, the process proceeds to step S43.
This registration-in-roaming flag is a single bit flag which can be
set or reset by the user to specify whether to enable or disable
the PLA setting unit 1d when the mobile station 1 is roaming. If
this flag is set, it means that the user wishes the PLA setting
function to work even in roaming mode.
(S42) The location change counting unit 1c determines whether its
location change counting table is valid. If it is valid, the
process advances to step S45, and otherwise, the process proceeds
to step S44. The location change counting table is regarded as
valid if, for example, it has collected data for at least one
month.
(S43) The location registration unit 1g performs an ordinary
location registration process, using a network default location
area which is defined as a default location area inherent to the
system.
(S44) The location change counting unit 1c adds one to a relevant
item of the location change counting table.
(S45) The PLA setting unit 1d advances the process to step S47 if
the personal location areas have already been defined. If not, then
it executes step S46.
(S46) The PLA setting unit 1d defines personal location areas,
based on the location change counting table, according to the steps
described in FIGS. 4 and 8.
(S47) The PLA setting unit 1d determines whether the personal
location areas are ready to operate. If ready, the process advances
to step S48, and otherwise, the process branches to step S43. More
specifically, the process advances to step S48 if at least one
personal location area is defined in the PLA table. If no personal
location area is defined, the process goes to step S43.
(S48) The location registration unit 1g obtains a current location
record of the mobile station 1 from an internal memory or register
of the location registration unit 1g. What is obtained here as the
"current location record" is actually an identifier which specifies
either a personal location area or a network default location area
provided by the system.
(S49) The location registration unit 1g exits from the present
process after executing a "Location Registration" routine. The
details of this routine will be described in the next paragraph
with reference to FIG. 11.
FIG. 11 is a flowchart which shows the details of the "Location
Registration" routine called at step S49. Note that the current
location record is passed to this routine as its argument, or input
parameter. The routine comprises the following steps.
(S60) The location registration unit 1g determines whether the
given argument, i.e., current location record obtained at step S48
(FIG. 10), is a personal location area. If it is, the process
advances to step S61, and otherwise, the process branches to step
S67.
(S61) The location registration unit 1g determines whether the
current base station ID (BS-ID) extracted by the base station ID
extractor is found in the personal location area obtained at step
S48 (FIG. 10). If the ID is found, then the control is returned to
the calling process, without sending any location update messages.
If not found, the process is directed to step S62 because it means
that the mobile station 1 has left the personal location area that
has so far been recognized as the current location.
(S62) The location registration unit 1g searches the PLA memory 1e
for another personal location area that includes the present base
station ID as a member.
(S63) If a relevant personal location area is found at step S62,
the location registration unit 1g then proceeds to step S64 to
update the location. If no relevant area is found, it goes to step
S65.
(S64) Searching the PLA memory 1e, the location registration unit
1g retrieves all base station IDs belonging to the new personal
location area, and sends them to the mobile switching center 2 via
the nearest base station, together with the identifier of the new
personal location area itself. FIG. 12 shows an example of a
location update message to be sent from the mobile station 1 to the
mobile switching center 2, which comprises the following data
items: (a) mobile identifier of the mobile station 1; (b) personal
location area ID, i.e., the identifier of the newly selected
personal location area; and (c) base station IDs indicating which
cells constitute the area. If the same personal location area has
been sent in the past, there is no need to send its base station
IDs again. If this is the case, the location registration unit 1g
can use an alternative message format shown in FIG. 13, eliminating
base station ID fields. Upon receipt of those messages, the mobile
switching center 2 extracts the new personal location area ID and
its associated base station IDs (if available), so that the PLA
configuration data memory 2a is updated with the new location
information.
(S65) The location registration unit 1g extracts the current
network default location area ID from the notification message that
the mobile station 1 has received. Network default location areas
are predefined groupings of cells provided by the system regardless
of mobility patterns of individual subscribers. Each base station
periodically sends its relevant area ID to mobile stations, using a
data field of the notification message as explained earlier in FIG.
6.
(S66) The location registration unit 1g performs location
registration by sending a location update message with the
extracted network default location area ID, and then the control is
returned to the calling process.
(S67) The location registration unit 1g searches the PLA memory 1e
for a relevant personal location area which contains the current
base station ID.
(S68) If a relevant personal location area is found at step S67,
the location registration unit 1g then proceeds to step S64 to
update the location. If no such area is found, it goes to step
S69.
(S69) The location registration unit 1g determines whether the
current location record agrees with the location area extracted
from the latest notification message. If they agree with each
other, then the control is returned to the calling process, without
sending any location update messages. Otherwise, the process has to
advance to step S70 because the current location record has turned
out to be obsolete.
(S70) The location registration unit 1g performs location
registration by sending a location update message with the network
default location area ID extracted from the notification message,
and the control is returned to the calling process.
As described above, the present invention permits location areas to
be defined in accordance with the mobility pattern of each
individual subscriber. This is accomplished by performing the
following steps of: (a) recording location changes as location
change count data for a predetermined period, (b) extracting, from
the location change count data, such count values exceeding a
predetermined threshold, and (c) defining personal location areas
by merging adjacent cells associated with the extracted count
values.
In the above-described embodiment, the location change counting
table is formulated from event records that were collected during a
continuous one-month period in order to define personal location
areas. It is, however, not intended to limit the invention to this
specific configuration, but an alternate idea described below is
considered to fall within the scope of the present invention. That
is, the time axis is divided into several appropriate periods, and
location change events are counted separately for those periods,
thereby yielding several location change counting tables. Personal
location areas are then defined independently for each specific
time period. This feature enables different sets of personal
location areas to be used on weekdays and weekend, for instance. By
introducing the concept of time segments as such, it becomes
possible to provide personal location areas that precisely reflect
the day-to-day mobility pattern of each individual subscriber.
While no particular access methods are mentioned so far, the
present invention can be implemented, for example, in a code
division multiple access (CDMA) network. The next paragraph will
provides some notes which may be helpful when implementing the
present invention in a narrowband-CDMA (N-CDMA) based system. The
specifications of N-CDMA are available as the CDMA mobile telephone
system standard, STD-T53 version 1.0, from the Association of Radio
Industries and Businesses (ARIB).
When the present invention is applied to N-CDMA systems, system
parameter messages can serve as the notification messages since
they contain a base station ID (BASE-ID). This BASE-ID allows each
base station to be uniquely identified within a network if it is
supported by a single network service provider. However, in a
heterogeneous environment where the network consists of a plurality
of sub-networks of different providers, it will be necessary to use
more identifiers, such as network IDs (NIDs) and system IDs (SIDs),
in combination with the base station IDs (BASE-IDs).
Further, it is possible to implement the location registration
messages by using "Registration Messages," which are originally
defined for the purpose of registering access channels. More
specifically, the personal location area ID and base station IDs
(FIG. 12 or 13) can be sent to the mobile switching center 2, being
added to a registration message.
The above discussion is now summarized as follows. According to the
present invention, each mobile station comprises: a notification
message receiver, a base station ID extractor, a location change
counting unit, a PLA setting unit, a PLA memory, a location change
detector, and a location registration unit, The notification
message receiver receives a notification message from the nearest
base station, and from this message, the base station ID extractor
extracts a base station ID to identify in which cell the mobile
station is currently located. The location change counting unit
detects a change in the mobile location by comparing two
consecutive instances of the base station ID, and it counts and
records such location changes, together with the two base station
IDs involved in each change. Based on those location change count
values, the PLA setting unit defines personal location a areas by
grouping associated cells into one area. The PLA memory stores the
defined personal location areas and relevant base station IDs. Each
time a new base station ID is detected, the location change
detector tests whether the presently registered personal location
area is still valid. If it is no longer valid, the location
registration unit searches the PLA memory for a new relevant
personal location area. The location registration unit then sends a
location updating message to notify the mobile switching center of
the new personal location area and its relevant base station IDs.
The mobile switching center accepts the location registration from
mobile stations. That is, its PLA configuration data memory stores
the content of each received location updating message. The mobile
switching center further comprises a paging unit, which uses the
PLA configuration data memory to page a specific mobile station
through a group of base stations belonging to the current personal
location area of the mobile station.
The mobile communications system configured as above will provide
for an advantageous mechanism to reduce the control message traffic
effectively in paging and location updating operations. This
arrangement also permits mobile stations to be paged promptly,
without wasting time or bandwidth.
The foregoing is considered as illustrative only of the principles
of the present invention. Further, since numerous modifications and
changes will readily occur to those skilled in the art, it is not
desired to limit the invention to the exact construction and
applications shown and described, and accordingly, all suitable
modifications and equivalents may be regarded as falling within the
scope of the invention in the appended claims and their
equivalents.
* * * * *